The present invention relates to a thyristor firing circuit utilizing a light actuated silicon controlled rectifier which has improved immunity to electrical noise.
Thyristors are commonly used to control the power supplied to a load from an a.c. source as by connecting the thyristors in an intermediate bridge.
Recent developments in optoelectronic devices, such as light emitting diode--light actuated silicon controlled rectifier packages have made their use highly advantageous in thyristor firing circuitry. These advantages include complete isolation between the power and control circuits, precise control of firing, small physical size and others. However, the silicon controlled rectifier of such light coupled devices is subject to spurious operation responsive to the application of transient voltages having a high rate of change to the anode-cathode circuit. Such voltage spikes or surges typically characterize electrical noise in the a.c. lines.
It is therefore, the object of the present invention to provide an improved light actuated silicon controlled rectifier thyristor firing circuit having improved immunity to noise.
Briefly, the present invention is utilized in a power control circuit having at least one thyristor with its power circuit connected to a load and to a.c. buses energized by an a.c. power source. A firing circuit for the thyristor includes a light actuated controlled rectifier providing firing pulses to the photo sensitive area of the thyristor. A resistor-capacitor circuit, connected across the buses, provides a voltage reduced with respect to the voltage in the buses for application to the anode of the SCR as an energizing voltage. The voltage reducing and capacitance properties of the resistor-capacitor circuit suppress transient voltage spikes in the a.c. buses, thereby rendering the operation of the controlled rectifier immune to such phenomena. The resistor-capacitor circuit typically includes two series connected resistor-capacitor pairs, each of the pairs being connected in series with each other across the a.c. buses.